Horizontal directional drilling (HDD) operations are used in drilling for utilities such as water and telephone lines. In HDD, the boreholes are shallow and typically extend under roads, rivers and other obstacles. To drill the borehole, a drill string is equipped with a drill bit. The drill string is rotated and forced through the ground. Fluid in the form of water or drilling mud is circulated through the drill stem, out the drill bit and back to the surface on the outside of the drill stem.
Drill stems or strings typically contain a sonde. The sonde is located near the drill bit and transmits a signal. One such sonde is shown and described in U.S. Pat. No. 5,155,442. An operator on the surface above the drill bit has a receiver and can receive the signal. Sonde information is used to guide and steer the drill bit and consequently guide and steer the borehole.
The sonde requires electrical power to operate. In the prior art, this power is generated by one of seven ways.
One of the primary ways to supply power downhole is simply through the use of batteries. This system is used in some of the sondes offered by Digital Control Inc. or Charles Machine Works. These batteries have a lifespan that varies, but a typical lifespan is less than 20 hours. The problem with this is that these batteries often fail during the drilling operation. Pulling the drill stem out of the bore and replacing the batteries increases the cost of drilling. Also, batteries need frequent changing requiring operator time to access the sonde. Also once these batteries are “used up” they are thrown away, contributing to a more toxic environment.
A second way to supply power to the electrical components downhole is to thread a conductive wire through the center of the drill stem. This method is known as a wireline system. This wire supplies electrical power from a power source on the surface. In order to use this system the wire has to be extended through each drill stem as the bore is lengthened. This is done by connecting additional lengths of wire in the I. D. of the stem and then encasing the connection in a protective wrap. U.S. Pat. No. 5,577,560 refers to this type system. This system is very time consuming and cannot be done on some drill rigs.
A third power supply system uses impellers rotated by the flow of drilling fluid. U.S. Pat. Nos. 7,165,608 and 7,133,325 show this type generating system. A simple generator is sealed off from the drilling fluid while its rotor is turned by the flow of drilling fluid. This system is relatively expensive to produce and is subject to break downs because of the corrosive nature of the drilling fluids.
A fourth way of generating electrical power is disclosed in U.S. Pat. Nos. 6,857,484 and 5,957,222. These systems have a generator that is lateral to the drill stem and engaged with the drill by gears. As the drill stem rotates, the generator produces power. These systems are relatively expensive.
A fifth way to generate power downhole is to use a dual drill stem system as does Charles Machine Works as described in U.S. Pat. Nos. 6,857,484 and 7,025,152. This system utilizes a drill string inside of a drill string extended to the surface to activate the elements of a typical generator. Again this system is quite expensive.
A sixth way is a linear generator which is included in a shock absorber together with the other subsurface components. Details of the linear generator included in a shock absorber can be found in U.S. Pat. No. 3,448,305. This system is expensive and very unwieldy in a drill string.
A seventh system uses responsive means that uses a piezo-electric disc connected to rectifying and smoothing circuits whereby a D.C. output is obtained. For example, U.S. Pat. No. 3,970,877 discloses a method for generating downhole electric energy using a means responsive to turbulence in the drilling mud flow to convert vibratory motion into an electrical output. This system does not produce an adequate amount of power.